A. Field of the Invention
The present invention relates to a new and improved trim design for a control valve that provides energy loss and high resistance to fluid flow by way of collision and separation of the fluid stream thereby preventing cavitation.
B. Description of the Background Art
Cavitation occurring in control valves has a damaging effect resulting in removal of material from internal valve surfaces. Cavitation occurs when pressure at the controlling orifice drops below vapor pressure causing vapor bubbles to form. Beyond the controlling orifice where pressure increases above vapor pressure, the vapor bubbles collapse. Internal portions and components of the valve near the collapsing bubbles suffer cavitation damage. To eliminate cavitation, it is necessary to prevent the pressure at the controlling orifice from dropping below vapor pressure level. This can be accomplished by staging the single valve pressure drop into multiple pressure drops thereby lowering the amount of pressure recovery occuring at any one stage.
There have been three basic techniques employed by valve designers to control or eliminate damage resulting from cavitation. A first technique involves hardening the internal valve surfaces in areas where cavitation may occur. A second technique employs flow-to-close trim designs that direct cavitating fluid jet streams to converge on one another at mid-stream locations thereby causing the vapor bubbles to collapse at locations away from critical internal surfaces and components of the valves. Both of these techniques attempt to minimize and contain the damaging effects of cavitation but do not eliminate cavitation. Consequently, these techniques are normally used for lower pressure drop applications where cavitation is not as severe as in other applications.
A third techique is one of cavitation elimination. Downstream restrictors are used to backup the outlet pressure of a control valve. This technique necessitates the use of staging external to the control valve resulting in expensive and bulky valves.
A trim assembly providing resistance to fluid flow by directing fluid through axially extending fluid energy absorbing passages is disclosed in U.S. Pat. No. 3,971,411. Radial flow passages defined by slots in one or more cylinders is illustrated in U.S. Pat. Nos. 3,813,079 and 3,987,809. A complex and cumbersome assembly for imparting resistance to flow is illustrated in U.S. Pat. No. 3,780,767 that discloses a plurality of plates with cut-out portions defining vortex chambers. Each of the designs disclosed in the above mentioned patents provides a single technique of resisting fluid flow. Each uses either radial or axial flow resistance and are primarily intended for noise abatement. These devices typically do not break the single large valve pressure drop into a number of smaller pressure drop stages to avoid creating vapor pressure bubbles in the fluid controlled by the valve.